Diffraction Based Overlay Metrology Tool and Method of Diffraction Based Overlay Metrology

ABSTRACT

Systems, methods, and apparatus are provided for determining overlay of a pattern on a substrate with a mask pattern defined in a resist layer on top of the pattern on the substrate. A first grating is provided under a second grating, each having substantially identical pitch to the other, together forming a composite grating. A first illumination beam is provided under an angle of incidence along a first horizontal direction. The intensity of a diffracted beam from the composite grating is measured. A second illumination beam is provided under the angle of incidence along a second horizontal direction. The second horizontal direction is opposite to the first horizontal direction. The intensity of the diffracted beam from the composite grating is measured. The difference between the diffracted beam from the first illumination beam and the diffracted beam from the second illumination beam, linearly scaled, results in the overlay error.

This patent application is related to U.S. application Ser. No.13/676,562, U.S. application Ser. No. 12/747,795, InternationalApplication No. PCT/NL08/50785 and U.S. Provisional Patent Application61/006,073, which are incorporated by reference herein in theirentireties.

FIELD

The present invention relates to a diffraction based overlay metrologytool and method of diffraction based overlay metrology.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.including part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

For lithographic processing, the location of patterns in subsequentlayers on the substrate should be as precise as possible for a correctdefinition of device features on the substrate, which features allshould have sizes within specified tolerances. The overlay error (i.e.,the mismatch between subsequent layers) should be within well-definedtolerances for creating functional devices.

To this end, an overlay measurement module is generally used fordetermining the overlay error of a pattern on the substrate with a maskpattern as defined in a resist layer on top of the pattern.

The overlay measurement module typically performs the measurement withoptics. The position of the mask pattern in the resist layer relative tothe position of the pattern on the substrate is determined by measuringan optical response from an optical marker which is illuminated by anoptical source. The signal generated by the optical marker is measuredby a sensor arrangement. Using the output of the sensors the overlayerror can be derived. Typically, the patterns on which overlay error aremeasured are located within a scribe lane in between target portions.

Two basic concepts are known for overlay metrology.

A first concept relates to measurement of overlay error that is imagebased. A position of an image of the pattern on the substrate iscompared to the position of the mask pattern in the resist layer. Fromthe comparison the overlay error is determined. An example to measureoverlay error is the so-called box-in-box structure, in which theposition of an inner box within an outer box is measured relative to theposition of the outer box.

Image based overlay error measurement may be sensitive to vibrations andalso to the quality of focus during measurement. For that reason, imagebased overlay error measurement may be less accurate in environmentsthat are subjected to vibrations, such as within a track system. Also,image-based overlay measurements may be susceptible to aberrations inthe optics that may further reduce the accuracy of the measurement.

A second concept relates to measurement of overlay error that isdiffraction based. In the pattern layer on the substrate a first gratingis located, and in the resist layer a second grating is located with apitch that is, substantially identical to the first grating. The secondgrating is located nominally on top of the first grating. By measuringthe intensity of the diffraction pattern as generated by the first andsecond grating superimposed on each other, a measure for the overlayerror may be obtained. If some overlay error is present between thefirst and second grating, this is detectable from the diffractionpattern.

In diffraction based overlay error measurement, only the first andsecond gratings may be illuminated, since light that reflects fromadjacent regions around the gratings interferes with the intensity levelof the diffraction pattern. However, a trend emerges to have overlayerror measurements close to critical structures within a die (and notnecessarily within the scribe lane). Also, there is a demand to reducethe size of gratings so as to have a larger area available forcircuitry. To some extent, such demands can be accommodated by areduction of the cross section of the illumination beam that impinges onthe first and second gratings so as to avoid illumination of the regionoutside the gratings. However, the minimal cross-section of theillumination beam is fundamentally limited by the laws of physics (i.e.limitation due to diffraction). Below, the cross-sectional size in whichdiffraction of the beam occurs will be referred as the diffractionlimit.

SUMMARY

It is desirable to have an improved diffraction based overlay errormeasurement system and method.

According to an aspect of the invention, there is provided a method fordetermining overlay error between a first pattern on a surface of asubstrate and a second pattern superimposed on the first pattern, thesubstrate comprising a first grating in the first pattern and a secondgrating on top of the first grating, the second grating havingsubstantially identical pitch as the first grating, the second and firstgratings forming a first composite grating, the method including:providing a first illumination beam for illuminating at least the firstcomposite grating under an angle of incidence along a first horizontaldirection along the surface of the substrate, the substrate being in afixed position, and measuring a first intensity of a first orderdiffracted beam from the first composite grating; and providing a secondillumination beam for illuminating at least the first composite gratingunder the angle of incidence along a second horizontal direction alongthe surface of the substrate, wherein the second horizontal direction isopposite to the first horizontal direction, the substrate being in thefixed position, and measuring a second intensity of a negative firstorder diffracted beam from the first composite grating.

According to an aspect of the invention, the method further includesdetermining an intensity difference between the first intensity and thesecond intensity, the intensity difference being proportional to theoverlay error between the first grating and the second grating.

According to an aspect of the invention, the first and secondillumination beams are portions of a common illumination beam.

According to an aspect of the invention, the common illumination beamhas an annular cross-section.

According to an aspect of the invention, the angle of incidence isoblique relative to the surface of the substrate, the diffraction angleof the first and negative first diffraction beam relative to the normalof the surface being smaller than the angle of incidence.

According to an aspect of the invention, the angle of incidence issubstantially perpendicular to the surface of the substrate, and themethod includes using the first illumination beam as the secondillumination beam, and the measuring of the first intensity of the firstorder diffracted beam from the first composite grating and of the secondintensity of the first order diffracted beam from the first compositegrating being performed consecutively during provision of the firstillumination beam.

According to an aspect of the invention, the method includes: blockingbeams of diffraction order other than the first diffraction order whenproviding the first illumination beam; blocking beams of diffractionorder other than the negative first diffraction order when providing thesecond illumination beam.

According to an aspect of the invention, the measuring of the firstintensity of the first order diffracted beam from the composite gratingincludes: detecting an image of the composite grating obtained by onlythe first order diffracted beam by pattern recognition, and themeasuring of the second intensity of the composite grating obtained byonly the negative first order diffracted beam from the composite gratingincludes: detecting an image of the composite grating obtained by onlythe negative first order diffracted beam by pattern recognition.

According to an aspect of the invention, the method includes providing asecond composite grating on the substrate, the second composite gratingbeing formed by a third grating in the first pattern and a fourthgrating on top of the first grating, the third grating and fourth havingsubstantially identical pitch as the first and second grating, in whichthe first composite grating is biased with a first shift in a shiftdirection along the grating direction and the second composite gratingis biased with a second shift in the shift direction along the gratingdirection, the first shift being different from the second shift;providing the first illumination beam for illuminating the secondcomposite grating under an angle of incidence along the first horizontaldirection along the surface of the substrate, the substrate being in thefixed position, and measuring a first intensity of a first orderdiffracted beam from the second composite grating; providing the secondillumination beam for illuminating the second composite grating underthe angle of incidence along the second horizontal direction along thesurface of the substrate, and measuring a second intensity of a negativefirst order diffracted beam from the second composite grating.

According to an aspect of the invention, there is provided a detectionsystem configured to determine overlay error between a first pattern ona surface of a substrate and a second pattern superimposed on the firstpattern, including an illumination source, a plurality of lenses, anaperture stop and an image detector, the plurality of lenses beingarranged along an optical path between a substrate position for holdinga substrate and the image detector; the substrate including a firstgrating in the first pattern and a second grating on top of the firstgrating, the second grating having identical pitch as the first grating,the second and first gratings forming a composite grating; theillumination source being arranged to form a first illumination beam forilluminating the composite grating on the substrate under an angle ofincidence along a first horizontal direction along the surface of thesubstrate, the substrate being in the substrate position; the imagedetector being arranged to receive a first order diffracted beam fromthe composite grating; the illumination source being arranged to form asecond illumination beam for illuminating the composite grating on thesubstrate under an angle of incidence along a second horizontaldirection along the surface of the substrate wherein the secondhorizontal direction is opposite to the first horizontal direction, thesubstrate being in the substrate position, the image detector beingarranged to receive a negative first order diffracted beam from thecomposite grating.

According to an aspect of the invention, there is provided alithographic apparatus including a detection system for determiningoverlay error between a first pattern on a surface of a substrate and asecond pattern superimposed on the first pattern as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIGS. 2 a, 2 b, 2 c illustrate overlay error metrology based ondiffraction according to an embodiment;

FIGS. 3 a, 3 b depict a diffraction based overlay error detection systemin accordance with an embodiment of the present invention, during afirst measurement and a second measurement respectively;

FIG. 4 a illustrates exemplary measurements of intensity of a negativefirst order and first order diffracted beams as function of overlayerror;

FIG. 4 b illustrates a difference of intensity between the negativefirst order and first order diffracted beams as function of overlayerror; and

FIG. 5 depicts a correlation between image based overlay error and thediffraction based overlay error as determined according to the presentinvention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus includes an illuminationsystem (illuminator) IL configured to condition a radiation beam B (e.g.UV radiation or EUV radiation); a patterning device support or a supportstructure (e.g. a mask table) MT constructed to support a patterningdevice (e.g. a mask) MA and connected to a first positioner PMconfigured to accurately position the patterning device in accordancewith certain parameters; a substrate table (e.g. a wafer table) WTconstructed to hold a substrate (e.g. a resist-coated wafer) W andconnected to a second positioner PW configured to accurately positionthe substrate in accordance with certain parameters; and a projectionsystem (e.g. a refractive projection lens system) PS configured toproject a pattern imparted to the radiation beam B by patterning deviceMA onto a target portion C (e.g. including one or more dies) of thesubstrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The patterning device support or support structure holds the patterningdevice in a manner that depends on the orientation of the patterningdevice, the design of the lithographic apparatus, and other conditions,such as for example whether or not the patterning device is held in avacuum environment. The patterning device support or support structurecan use mechanical, vacuum, electrostatic or other clamping techniquesto hold the patterning device. The support structure may be a frame or atable, for example, which may be fixed or movable as required. Thesupport structure may ensure that the patterning device is at a desiredposition, for example with respect to the projection system. Any use ofthe terms “reticle” or “mask” herein may be considered synonymous withthe more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the patterning device (e.g. mask) and the projection system.Immersion techniques are well known in the art for increasing thenumerical aperture of projection systems. The term “immersion” as usedherein does not mean that a structure, such as a substrate, must besubmerged in liquid, but rather only means that liquid is locatedbetween the projection system and the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as a-outer anda-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL mayinclude various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the patterning device support or support structure(e.g., mask table MT), and is patterned by the patterning device. Havingtraversed the patterning device (e.g. mask) MA, the radiation beam Bpasses through the projection system PS, which focuses the beam onto atarget portion C of the substrate W. With the aid of the secondpositioner PW and position sensor IF (e.g. an interferometric device,linear encoder or capacitive sensor), the substrate table WT can bemoved accurately, e.g. so as to position different target portions C inthe path of the radiation beam B. Similarly, the first positioner PM andanother position sensor (which is not explicitly depicted in FIG. 1) canbe used to accurately position the patterning device (e.g. mask) MA withrespect to the path of the radiation beam B, e.g. after mechanicalretrieval from a mask library, or during a scan. In general, movement ofthe patterning device support (e.g. mask table) MT may be realized withthe aid of a long-stroke module (coarse positioning) and a short-strokemodule (fine positioning), which form part of the first positioner PM.Similarly, movement of the substrate table WT may be realized using along-stroke module and a short-stroke module, which form part of thesecond positioner PW. In the case of a stepper (as opposed to a scanner)the patterning device support (e.g. mask table) MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device (e.g.mask) MA and substrate W may be aligned using mask alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device (e.g. mask) MA, themask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the patterning device support (e.g. mask table) MT andthe substrate table WT are kept essentially stationary, while an entirepattern imparted to the radiation beam is projected onto a targetportion C at one time (i.e. a single static exposure). The substratetable WT is then shifted in the X and/or Y direction so that a differenttarget portion C can be exposed. In step mode, the maximum size of theexposure field limits the size of the target portion C imaged in asingle static exposure.

2. In scan mode, the patterning device support (e.g. mask table) MT andthe substrate table WT are scanned synchronously while a patternimparted to the radiation beam is projected onto a target portion C(i.e. a single dynamic exposure). The velocity and direction of thesubstrate table WT relative to the patterning device support (e.g. masktable) MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PS. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the patterning device support (e.g. mask table) MTis kept essentially stationary holding a programmable patterning device,and the substrate table WT is moved or scanned while a pattern impartedto the radiation beam is projected onto a target portion C. In thismode, generally a pulsed radiation source is employed and theprogrammable patterning device is updated as required after eachmovement of the substrate table WT or in between successive radiationpulses during a scan. This mode of operation can be readily applied tomaskless lithography that utilizes programmable patterning device, suchas a programmable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIGS. 2 a, 2 b, 2 c illustrate the interaction between an illuminationbeam and 2 overlapping gratings that may be used for diffractive overlaymetrology according to an embodiment.

In FIG. 2 a a cross section of a composite grating 110, 120 is shownwhich exhibits a zero overlay error.

On a substrate 100 a composite grating is constructed which includes ofa first grating 110 and a second grating 120. The first grating 110 ispatterned in the substrate material and includes a first periodicstructure along a grating direction X1.

In an embodiment, the periodic structure of the first grating includes aplurality of primary lines 111 with secondary lines 112 interposed. Theperiodic structure is formed in layer 115.

For reason of clarity, only one primary line 111 and one adjacentsecondary line 112 have been indicated by reference numbers in FIG. 2 a.

The pitch P of the grating 110 is equal to the width of one line 111 andone secondary line 112.

As will be appreciated by the skilled in the art, the secondary lines112 may be created from trenches in between the substrate lines 111 thatare filled by a material different from the substrate material. Forexample, the substrate material is silicon and the trench material is adielectric like silicon dioxide, or a metal like tungsten or copper.

On top of the first grating 110, a second grating 120 is present. Thesecond grating consists of a second periodic structure.

In the embodiment shown, the second periodic structure includesplurality of lines 121 with trenches 122 interposed along the gratingdirection X1.

In this example, lines 121 are positioned on top of the secondary lines112 of the first grating 110. The second grating 120 has a pitch P′ indirection X1 equal to the width of one line 121 and one trench 122. Thepitch P′ of the second grating 120 is chosen to be substantially equalto the pitch P of the first grating 110. In an embodiment, lines 121 ofthe second grating 120 may have substantially the same width as thesecondary lines 112 of the first grating 110.

Alternatively, lines 121 of the second grating 120 may be located on topof the primary lines 111 of the first grating 110.

The second grating may be a pattern formed in a resist layer 125.

In the case of FIG. 2 a, the alignment of the first and second gratings110 and 120 is perfect, the mismatch is ideally zero (which will bereferred to as a zero overlay error). The lines 121 of the secondgrating 120 are aligned fully with the secondary lines 112 of the firstgrating 110.

In FIG. 2 a, an embodiment for the interaction between an illuminationbeam IB and the composite grating 110, 120 is schematically shown.

In this embodiment, the illumination beam IB impinges under a firstoblique incidence angle β on the grating structure in grating directionX1. The angle of incidence β is taken relative to the surface normal n.The illumination beam IB is scattered by the composite grating 110, 120and forms (at least) two diffracted beams B+ and B0 of first order andzeroth order respectively, The first order diffracted beam B+ leaves thesubstrate under an angle θ (relative to the surface normal n) and thezeroth order diffracted beam leaves under specular reflection,respectively. It is noted that the pitch P of the composite grating 110,120 and the wavelength of the illumination beam IB are chosen so as tofulfill the diffraction condition. In FIG. 2 a, the diffraction ordersand illumination beam are shown in one plane but this is just forconvenience. An embodiment of the invention is also applicable in caseof conical diffraction where the diffracted beams may not be in the sameplane as the illumination beam.

Depending on the ratio of the grating's pitch P (with P=P′) and thewavelength of the illumination beam IB also higher order diffractedbeams may be present but these are ignored here.

In FIG. 2 b, the same cross section of the composite grating 110, 120 ofFIG. 2 a for a second oblique incidence of the illumination beam IB.

In FIG. 2 b, the illumination beam IB impinges under a second obliqueincidence angle −β on the grating structure. Second incidence angle −βhas substantially the same magnitude as the first incidence angle β, butis, in comparison, directed in an opposite direction along gratingdirection X1. The second angle of incidence −β is taken relative to thesurface normal n.

The illumination beam IB is scattered by the composite grating 110, 120and forms (at least) two diffracted beams B− and B0 of first (negative)order and zeroth order respectively, which leave the substrate under anangle −θ and under specular reflection, respectively.

Diffracted beam B+ depicts the first diffraction order, diffracted beamB− depicts the negative first diffraction order. Due to the fact thatthe first and second grating are fully aligned, the composite grating issymmetrical, i.e. the secondary lines 112 of the first grating 110coincide with the lines 121 of the second grating 120 as composite lines112, 121. As a result of the symmetry of the composite grating, thediffraction pattern is also symmetrical: i.e., an intensity I+ of thefirst order diffracted beam B+ is substantially equal to an intensity I−of the negative first order diffracted beam B−.

I+=I−=I ₊₀   eq. (1),

wherein I₊₀ denotes the intensity of first order diffracted beam for thesymmetric composite grating.

In FIG. 2 c, a cross section of a composite grating 110, 120 is shownwhich exhibits a non-zero overlay error. The lines 121 of the secondgrating 120 display an overlay error (misalignment) ε relative to thesecondary lines 112 of the first grating. As a result, the compositegrating as shown in FIG. 2 c is asymmetrical: the lines 121 of thesecond grating 120 are shifted over a distance ε in comparison with thesecondary lines 112 in the first grating 110.

Due to the asymmetry, the intensity I+ of the first order diffractedbeam B+ measured under first oblique incidence angle β is in this casenot equal to the intensity I− of the negative first order diffractedbeam B−, measured under second oblique angle −β.

For small overlay errors, the change of intensity of a diffracted beamis linearly proportional to the overlay error. The intensity I+ of thefirst order diffracted beam B+ as a function of overlay error ε is ingood approximation:

I+=I ₊₀ +K×ε  eq. (2),

wherein K is a proportionality factor.

The intensity I− of the negative first order diffracted beam B− isapproximated by:

I−=I ₊₀ −K×ε  eq. (3)

By taking the difference ΔI=I+−I−, a signal is obtained that scaleslinearly with the overlay error ε.

ΔI=2K×ε  eq. (4)

The proportionality factor K will be discussed in more detail below.

In a further embodiment, overlay metrology may include a use of thefirst illumination beam IB1 and the second illumination beam IB2 eachunder substantially normal incidence on the composite grating 110, 120.It will be appreciated by the skilled person that in such an embodiment,the first illumination beam IB1 and the second illumination beam IB2coincide and are provided as a single illumination beam. The firstillumination beam can be used as second illumination beam. Under normalincidence of the illumination beam also, first and negative first orderdiffraction beams B+, B− will occur. Of these beams B+, B−, theintensity will show the same relationship as described above withreference to FIGS. 2 a-2 c and equations 1-4. In this embodiment, theintensity difference ΔI of the first and negative first order diffractedbeams may be measured by using the first illumination beam andconsecutively measuring the intensity of the first and negative firstorder diffracted beam, respectively.

FIG. 3 a depicts schematically a diffraction based overlay errordetection system (hereafter referred to as detection system) 200 inaccordance with an embodiment of the present invention, in a firstmeasurement of the substrate holding the composite grating 110, 120. Thedetection system may include a support configured to support thesubstrate in an embodiment of the invention. The support may also be asubstrate table of the lithographic apparatus of FIG. 1 in an embodimentof the invention.

The detection system 200 includes a plurality of lenses, in thisembodiment, a first, second, third and fourth positive lens L1, L2, L3,L4, an aperture stop DF, and an image detector ID.

Within the detection system 200, an optical axis OP is arranged thatextends from a substrate position where a composite grating 110, 120 canbe illuminated by the illumination beam IB under oblique incidence angleto a position where an image of the composite grating can be projectedon the image detector ID.

For example, the image detector ID may be a CCD camera. The illuminatedarea is larger than the area of the grating. In other words, thesurrounding environment is also illuminated. This is also referred to as“overfill”.

Along the optical axis OP, the first, second, third and fourth positivelenses L1, L2, L3, L4 are arranged with their respective centers on theoptical axis in such a way that the image of the composite grating 110,120 can be projected on the image detector ID of the detection system200.

The first lens L1 is positioned above the substrate position where thecomposite grating 110, 120 on the substrate 100 can be located. Thedistance between the first lens and the substrate position issubstantially equal to a focal distance F1 of the first lens L1. At somedistance from the first lens L1, the second and third lenses L2, L3 arearranged in a pair along the optical axis OP. The fourth lens L4 isarranged as projection lens of the image detector ID. Between the thirdand the fourth lenses L3, L4, the aperture stop DF is located.

During measurement, the substrate with composite grating 110, 120 islocated at the substrate position. The composite grating 110, 120 is ina predetermined position (indicated Q). A first illumination beam IB1 isused in an asymmetric illumination mode under oblique incidence in afirst horizontal direction (indicated by arrow D1) along the surface ofthe substrate. For example, the first illumination beam propagates alonga direction that has a component along a first horizontal directionalong the surface of the substrate. The first illumination beam IB1enters the first lens L1, in such way that the first illumination beamIB1 after passing the first lens impinges on the composite grating underan angle that creates a first diffraction order beam B+ underdiffraction angle θ. As a result, the first order diffracted beam B+ isnow diffracted at the surface of the substrate and a zeroth orderdiffraction beam B0 is diffracted under specular reflection (in thisexample under angle 2θ).

Both first order diffracted beam B+ and zeroth order beam B0 passthrough the first lens L1. Since the composite grating is at a focaldistance F1 of the first lens L1, the first order and zeroth orderdiffracted beams B+, B0 are directed in parallel after passing the firstlens L1.

Next, the first order and zeroth order diffracted beams B+, B0 pass thesecond lens L2. The first order diffracted beam B+ substantiallycoincides with the optical axis and passes through the center of thesecond lens L2. The zeroth order diffracted beam B0 passes the secondlens L2 off-axis and after passing is directed through the focal pointof the second lens L2.

The third lens L3 is arranged with a focal point F3 coinciding with afocal point F2 of the second lens L2.

The first order diffracted beam B+ coincides with the optical axis ofthe third lens and passes through the center of the third lens L3 andcontinues to be on the optical axis. The zeroth order diffracted beam B0passes the third lens off-axis. Due to the fact that the focal pointsF2, F3 of the second and third lenses coincide, the zeroth orderdiffracted beam is substantially parallel to the optical axis afterpassing the third lens L3.

After the third lens L3 the aperture stop DF is positioned on theoptical axis and is arranged to block the zeroth diffraction order. Theaperture stop DF allows the first order diffracted beam B+ on theoptical axis OP to pass and blocks the zeroth order diffracted beam B0.In this way, the image on the camera is only formed by first diffractionorder and not by the zeroth order. This imaging mode is normally called“dark-field” imaging. The aperture stop DF is arranged to have a widththat allows to block the zeroth order diffracted beam B0 and allows tolet the first order diffracted beam B+ pass.

As a result, an image of the composite grating is formed on the CCDcamera using only the first or negative first diffraction order.Suitable image processing and pattern recognition algorithms known tothe skilled artisan may then be used to identify the composite gratingfrom the product structures around the composite grating. Application ofthe aperture stop allows to use an illumination beam with across-sectional size larger than the diffraction limit, while the sizeof the grating may be smaller than indicated by the diffraction limit.

Finally, the first order diffracted beam B+ passes the fourth lens L4which is arranged for imaging the first order diffracted beam B+ on theimage detector ID.

In this manner, an image of the composite grating 110, 120 originatedfrom the first order diffracted beam B+ is projected on the imagedetector ID. Since the image is only formed by one higher (first)diffraction order, the image will show no modulation of the individualgrating lines.

It is noted that the first diffracted order may not necessarily beexactly normal to the surface. The first diffracted order may make anyangle with the wafer surface, as long as it is transmitted by theaperture stop (without any other orders passing the aperture stop).

From the image of the composite grating 110, 120 registered on the imagedetector, the intensity I+ may be determined. The precise location ofthe image of the grating is determined with pattern recognitionalgorithms, for example edge detection.

FIG. 3 b depicts schematically a diffraction based overlay errordetection system in accordance with an embodiment of the presentinvention, in a second measurement of the substrate holding thecomposite grating 110, 120.

In FIG. 3 b entities with the same reference number as shown in thepreceding figures refer to the corresponding entities.

In the second measurement, the composite grating 110, 120 is illuminatedasymmetrically by a second illumination beam IB2 in a second horizontaldirection (indicated by arrow D2) opposite to the first horizontaldirection D1 as used during the first measurement as shown in FIG. 3 a.For example, the second illumination beam propagates along a directionthat has a component along a first horizontal direction along thesurface of the substrate. The composite grating is maintained in thesame predetermined position Q as during the first measurement.

Under these conditions, the negative first order diffracted beam B− isnow diffracted normal to the surface of the substrate and the zerothorder diffraction beam B0 is diffracted under angle θ. The aperture stopDF is arranged to have a width that allows to block the zeroth orderdiffracted beam B0 and allows to let the negative first order diffractedbeam B− pass.

As a result, during the second measurement an image of the compositegrating 110, 120 originated from the negative first order diffractionbeam B− is projected on the image detector ID. From the image of thecomposite grating 110, 120 registered on the image detector ID, theintensity I− may be determined. Again, pattern recognition techniquesmay be used to identify the region on the CCD where the measurement ofthe intensity must be carried out.

It is noted that in a different embodiment, the illumination beam hassubstantially normal incidence. As will be appreciated by the skilled inthe art, this embodiment may use a different but functionally equivalentillumination/detection layout in which the function of the aperture stopto allow in a first instance only the first order diffracted beam and ina second instance only the negative first order diffracted beam to pass,would be the same.

Moreover it is noted that oblique incidence is not required but may bepreferred since it allows the use of gratings with a smaller pitch.

As described above, the difference of the intensity I+ of the firstorder diffracted beam B+ and the intensity I− of the negative firstorder diffracted beam B− is proportional to the overlay error εaccording to eq. 4. The proportionality factor K is dependent onprocessing conditions, wavelength of the illumination beam, diffractionangle and polarization. For a given combination of process, wavelength,diffraction angle and polarization, it is desirable to carry out acalibration of the proportionality factor, as will be appreciated by theskilled in the art.

In an embodiment of the invention, the proportionality factor K iscalibrated by determining the overlay error ε on two biased compositegratings on a substrate. Each biased composite grating has a respectivepredetermined built-in shift between the first grating 110 and thesecond grating 120. The two biased gratings are on the substrate in afixed position relative to each other.

The first biased composite grating has a first built-in shift +d in ashift direction along the grating direction X1. The second biasedcomposite grating has a second built-in shift −d, which is equal to butwith opposite sign than the first built-in shift, along the gratingdirection X1.

In case of an overlay error ε, the first biased composite gratingexhibits a total overlay error ε+d and the second biased compositegrating exhibits a total overlay error ε−d.

An intensity difference ΔI1 between the first and negative firstdiffraction orders on the first biased composite grating and anintensity difference ΔI2 between the first and negative firstdiffraction orders on the second biased composite grating is given by:

ΔI1=K×(ε+d)   eq. (5)

for the first biased composite grating and

ΔI2=K×(ε−d)   eq. (6)

for the second biased composite grating.

Elimination of K results in:

$\begin{matrix}{ɛ = {d\; \frac{{\Delta \; I\; 1} + {\Delta \; I\; 2}}{{\Delta \; I\; 1} - {\Delta \; I\; 2}}}} & {{eq}.\mspace{14mu} (7)}\end{matrix}$

In an embodiment, both first and second biased composite gratings can bemeasured at the same time by the detection system as shown in FIGS. 3 a,3 b. In that case the image detector ID registers an image from thefirst biased composite grating and an image of the second biasedcomposite grating at the same time. By using image processing softwarethe intensity of the image of the first biased composite grating and theintensity of the second biased composite grating can be determinedseparately. The overlay error ε can be calculated using equations(5)-(7).

Since the first and second illumination beams IB1, IB2 are each undergrazing incidence, light that would reflect off surface regions outsideof the composite grating(s) (i.e., product area), will not likely reachthe image detector ID through the system of first, second, third andfourth lenses L1, L2, L3, L4. In an embodiment of the present invention,the first and/or second illumination beam IB1, IB2 may have a largercross-section than the composite grating 110, 120 on the substratewithout causing interference between light reflected off the surfaceoutside the grating and light diffracted by the composite grating.

A large value of the numerical aperture of the aperture stop DF ispreferred since it allows a sharp transition between the compositegrating and the surrounding product area in which the composite gratingis embedded. Since at the same time, the aperture stop DF is arranged toblock the zeroth order diffracted beam B0, the numerical aperture of theaperture stop DF has an upper limit in which a compromise is obtainedbetween sufficient zeroth order diffracted beam suppression and asufficiently low cross-talk due to reflections from the product area. Itis feasible that this approach allows the use of composite gratings of asize of about 10×10 μm2.

It is noted that modeling software may allow to compute a layout ofproduct area and embedded composite grating(s) for which the cross-talkcan be minimized further. This approach may allow to design embeddedcomposite gratings with a size of about 4×4 μm2.

In an embodiment, the numerical aperture of the aperture stop DF isabout 0.7, while the numerical aperture of the first lens is about 0.95.

FIG. 4 a illustrates exemplary measurements of intensity of negativefirst order and first order diffracted beams as function of shift d on abiased grating.

In FIG. 4 a, the variation of the intensity I− of the negative firstorder diffracted beam B− and the intensity I+ of the first orderdiffracted beam B+ with the shift d are shown for a composite gratingwith pitch P=660 nm and a wavelength λ=700 nm of the illumination beam.It is observed that the change of the intensity I+, I− for shifts closeto 0 nm is substantially linear.

FIG. 4 b illustrates the difference of intensity between the negativefirst order and first order diffracted beams as function of shift d onthe biased grating as shown in FIG. 4 b. It is observed that the changeof the intensity difference ΔI for shifts close to 0 nm is substantiallylinear.

FIG. 5 depicts a correlation between image based overlay error and thediffraction based overlay error as determined according to the presentinvention.

For a number of samples, the shift d of the biased composite gratings asmeasured by diffraction based overlay error metrology is also measuredby image based overlay error metrology. In FIG. 5, a correlation isshown of the overlay measured by diffraction (along the vertical axis)and the overlay as measured by an image based method (along thehorizontal axis). A linear fit of the data is illustrated by the solidline. Within the error of the methods, the coefficient of the solid lineis unity. The correlation coefficient is over 0.99.

It is noted that the illumination beam IB as described above may be asingle beam. Alternatively, the illumination beam may have a shape ofhalf an annulus as its cross-section. In that case, the asymmetricillumination in FIG. 3 a may be done by one half of the annular beam,while the asymmetric illumination from the opposite direction as shownin FIG. 3 b is done by the other half of the annular beam.

The illumination beam IB may be created by a light source such as amonochromatic lamp, or a laser source. A laser source with relativelyhigh intensity may be used in case a short time is available formeasurements.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

What is claimed is:
 1. A method comprising: illuminating a compositegrating on a surface of a substrate under a normal angle of incidence,the composite grating comprising first and second superimposed gratingswith similar pitch; measuring, at an image plane, a first intensity of afirst diffracted beam from the composite grating; and measuring a secondintensity of a second diffracted beam from the composite grating at theimage plane.
 2. The method of claim 1, further comprising: determiningan intensity difference between the first intensity and the secondintensity, the intensity difference being proportional to the overlayerror between the first grating and the second grating.
 3. The method ofclaim 1, wherein: measuring the first intensity comprises using patternrecognition to detect an image of the composite grating obtained by apositive first order diffracted beam, and measuring the second intensitycomprises using pattern recognition to detect an image of the compositegrating obtained by a negative first order diffracted beam.
 4. Themethod of claim 3, further comprising: blocking diffraction orders otherthan the first diffraction order.
 5. The method of claim 1, wherein:measuring the first intensity comprises using pattern recognition todetect an image of the composite grating obtained by a zero orderdiffracted beam, and measuring the second intensity comprises usingpattern recognition to detect an image of the composite grating obtainedby a zero order diffracted beam.
 6. The method of claim 1, furthercomprising: illuminating an additional composite grating on thesubstrate under a normal angle of incidence, wherein: the additionalcomposite grating is formed by positioning a third grating and a fourthgrating on top of the first grating, the third grating and the fourthgrating having a substantially identical pitch as the first and thesecond grating, the composite grating is biased with a first shift in ashift direction along the grating direction, and the additionalcomposite grating is biased with a second shift in the shift directionalong the grating direction, the first shift being different from thesecond shift.
 7. The method of claim 6, further comprising: measuring,at the image plane, a third intensity of a third diffracted beam fromthe additional composite grating; and measuring a fourth intensity of afourth diffracted beam from the additional composite grating at theimage plane.
 8. A detection system comprising: an illumination sourceconfigured to direct an illumination beam to diffract from a compositegrating on a surface of a substrate under a normal angle of incidence,the composite grating comprising first and second superimposed gratingshaving a similar pitch; an image detector configured to receive, at animage plane, a first diffracted beam and a second diffracted beam fromthe composite grating; a lens arranged along an optical path between thesubstrate position and the image detector; and an aperture stop.
 9. Thedetection system of claim 8, wherein: the first diffracted beamcomprises a positive first order diffracted beam and the seconddiffracted beam comprises a negative first order diffracted beam, andthe image detector is configured to use pattern recognition to detect animage of the composite grating using the first order diffracted beam andthe negative first order diffracted beam.
 10. The detection system ofclaim 8, wherein the image detector is configured to use patternrecognition to detect an image of the composite grating using only azero order diffraction beam.
 11. The detection system of claim 8,wherein: the lens comprises an objective lens adjacent to the surface ofthe substrate and a projection lens adjacent to the image detector, theaperture stop is arranged along the optical path between the objectivelens and the projection lens, and the objective lens has a firstnumerical aperture value and the aperture stop has a second numericalaperture value, the second numerical aperture value being smaller thanthe first numerical aperture value.
 12. The detection system of claim 8,wherein the image detector is further configured to measure a firstintensity of the first diffracted beam and a second intensity of thesecond diffracted beam.
 13. The detection system of claim 8, wherein theimage detector is further configured to determine an intensitydifference between the first intensity and the second intensity, theintensity difference being proportional to an overlay error between thefirst grating and the second grating.
 14. A lithographic apparatuscomprising a detection system, comprising: an illumination sourceconfigured to direct an illumination beam to diffract from a compositegrating on a surface of a substrate under a normal angle of incidence,the composite grating comprising first and second superimposed gratingswith similar pitch on a surface of a substrate; an image detectorconfigured to receive, at an image plane, a first diffracted beam and asecond diffracted beam from the composite grating; a lens arranged alongan optical path between the substrate position and the image detector;and an aperture stop.
 15. The lithographic apparatus according to claim14, further comprising: an illumination system configured to condition abeam of radiation; a patterning device support configured to hold apatterning device, the patterning device configured to pattern the beamof radiation to form a patterned beam of radiation; a substrate tableconfigured to hold the substrate; and a projection system configured toprojected the patterned beam of radiation onto the substrate.
 16. Thelithographic apparatus according to claim 14, wherein: the firstdiffracted beam comprises a first order diffracted beam and the seconddiffracted beam comprises a negative first order diffracted beam, andthe image detector is configured to detect an image of the compositegrating using the first order diffracted beam and the negative firstorder diffracted beam by a pattern recognition method.
 17. Thelithographic apparatus according to claim 16, wherein the aperture stopis configured to block beams of diffraction order other than the firstdiffraction order and the negative first diffraction order.
 18. Thelithographic apparatus according to claim 14, wherein the image detectoris configured to detect an image of the composite grating using only azero order diffraction beam by a pattern recognition method.
 19. Thelithographic apparatus according to claim 14, wherein: the lenscomprises an objective lens adjacent to the surface of the substrate anda projection lens adjacent to the image detector, the aperture stop isarranged along the optical path between the objective lens and theprojection lens, and the objective lens has a first numerical aperturevalue and the aperture stop has a second numerical aperture value, thesecond numerical aperture value being smaller than the first numericalaperture value.
 20. The lithographic apparatus according to claim 14,wherein image detector is further configured to: measure a firstintensity of the first diffracted beam and a second intensity of thesecond diffracted beam; and determine an intensity difference betweenthe first intensity and the second intensity, the intensity differencebeing proportional to an overlay error between the first grating and thesecond grating.